In rotary wing aircraft, such as helicopters, and fixed wing aircraft, the fuselage, whose structure is essentially a hull, may be required to bear localized stresses requiring the presence of one or more frames to distribute these stresses inside the working structure. In the case of a helicopter, frames such as these, especially reinforcement frames, are usually used for fixing the engine or engines and the transmission units to the rotor on the top of the fuselage. In fixed wing or rotary wing aircraft, reinforcement frames may also be used to fix the resistant elements of the undercarriage.
As shown in detail in the document FR-A-2 539 701, the method is known for producing the fuselage of a helicopter and in particular the reinforcement frames of this fuselage made of a composite material constituted by carbon fibers embedded in a plastic mould.
In this document, each of the frames includes two vertical struts and two horizontal crosspieces whose extremities are fixed to the extremities of the vertical struts. The struts and the crosspieces comprise a casing made of a composite material encompassing a honeycomb-shaped material. More precisely, the casing of the vertical struts has a cross section in the form of a high-shaped cap closed to the outside by a composite strip, whereas the casing of the horizontal crosspieces is formed of two U-shaped sections disposed back-to-back and connected by two composite strips. The assembling between the lateral beams and the horizontal beams is effected by glueing by making the extremities of the vertical struts penetrate into the recessed sections of the extremities of the horizontal crosspieces.
A reinforcement frame embodied in this way presents a number of drawbacks. Thus, the conception described only makes it possible to embody frames whose shape is approximately rectangular and is not applicable to frames having a different shape, said shape being, for example, circular or polygonal. Furthermore, the segmenting into three elements of the upper portico-shaped section of the frame results in discontinuities in the transmission to the entire frame structure of localized stresses applied to the latter, for example at locations used for the fixing of the main transmission box or the resistant devices of the undercarriage. Moreover, the positioning of the braces bearing these localized stresses has not been proved to be satisfactory. Finally, the flanges of the vertical struts of the frame are suddenly interrupted when these struts are connected onto the lower beam, which increases the structural discontinuity of the frame.
In addition, the document U.S. Pat. No. 4,593,870 describes a structure designed to constitute the lower part of the fuselage of an aircraft, such as a helicopter. This structure includes two webs made of a composite material between which placed is an alveolar material. In the lower part of the structure, the webs present deformations constituting the starts of folding bucklings when the structure of the fuselage is compression-stressed in a vertical direction under the effect of an impact of this structure at high speed.